I kind of lean toward the idea that Tier 4 engines will become an expendable item. With adequate maintenance and keeping the Fluid full, they will run as well as most other diesels - though only on ultra low sulfur fuel, which we now have in CA but perhaps not yet everywhere else. But something else will come along - call it Tier 4a or 5 or whatever - in 5 or 6 years that will be even better, and by then might be simpler to use.

On-road diesels like lite-duty Mercedes simply add fluid fill to the oil change maintenance schedule. If the tank for some reason does run dry you get a limited number of starts before it quits (and if Onstar or the like is available probably calls home for a tow truck). Obvious why the latter part of that might not be good news for heavy-duty diesel users, though if they work it into the maintenance schedule in theory the shutdown should never happen. The commuter roads are going that way, but their fleets are considerably more captive to one maintenance site than the Class Is'. And of course "In theory that works in practice, but not in practice..."

I expect a dearth of diesel purchases from the Big Boys for a while. Unless EPA can be leaned on to allow replacements of older stuff (say, Tier 0-1) with Tier 3 for a while longer (that's still a good-sized emission reduction). Also, a lot more rebuilds.